Important considerations for spinal instrumentation used to promote spinal fusion are safe insertion, rigid fixation, and ease and adaptability of implantation. Historically, Harrington rods were an early type of spinal instrumentation used to stabilize the thoracic and the upper lumbar regions to obtain correction and stability for fusion. This instrumentation involved long rods attached to hooks, which were inserted over the bony arches (lamina) of the spinal canal. The instrumentation was subject to failure when the hooks either broke the lamina or became displaced and thus, the intended stabilization and correction was lost. Other rod systems have been developed which employ rod-screw connectors. One example is the TSRH.RTM. Spinal System available from Danek Medical Inc., 1800 Pyramid Place, Memphis, Tenn. 38132, (800) 93-DANEK. Even with current improved systems remains the challenge of attachment of the rods to the spine.
Other spinal instrumentation systems include the use of a longitudinal member, such as a plate, to interconnect a series of pedicle screws such as that disclosed in U.S. Pat. No. 5,129,899 issued to Small. In this system, the clinician must strive to place the series of pedicle screws into the bone in longitudinal alignment with as little lateral displacement and angular deflection as possible so that the plate may be attached to the series of pedicle screws in order to achieve a solid biomechanical relationship. However, due to variances in the spinal column or imperfect placement by the clinician, the screws are often inserted at an angle or must be placed laterally outside the longitudinally axis defined by the plate and other screws. While slight angular deviations of screw placement may be overcome by the use of contoured washers, larger angular deflections and the problems associated with lateral displacement of the pedicle screws cannot be overcome by the instrumentation disclosed in the Small patent. Moreover, the use of such contoured washers adds to the number of parts to be manipulated by the surgeon.
The development of the Kambin Offset Bolt (Smith & Nephews Richard, Inc.) for use with the device disclosed in the Small, et al. patent allows greater flexibility when the pedicle screws cannot be placed in the desired longitudinal relationship. The Kambin Bolt has a double threaded shank with an upper portion offset 6 mm from the axis of the lower portion. This offset bolt, however, has several drawbacks. First, insertion of the offset bolt requires a larger area of clearance to obtain insertion. This often leads to insertion difficulty when the bolt interferes with the spinal retractors, other elements of the spine, or other screws which have already been implanted. This interference can cause other implanted screws to loosen when impinged. In addition, once in place, the offset bolt can only overcome translational misalignment, but not angular misalignment of the pedicle screws.
U.S. Pat. No. 5,129,900 to Asher, et al. discloses a connector for pedicle screws and spinal rods. Lateral deviations may be cured by the use of connector members which include oblong openings. (Col. 4 lines 54-67). Like the offset screw taught by the Small patent, however, the device disclosed in the Asher patent cannot overcome the difficulties associated with angular misalignment.
Accordingly, a need has remained for devices which conveniently compensate for both translational and angular misalignment of bone fasteners relative to spinal devices such as rods and plates.